Rock drill machines are employed for drilling and excavating rock e.g. in underground mines, opencast quarries and on land construction sites. Known rock drilling and excavating methods include cutting, crushing and percussing methods. Percussion methods are most commonly in use in connection with hard rock types. In the percussion method the tool of the drill machine is both rotated and struck. Rock breaks, however, mainly by the effect of an impact. The main function of the rotation is to make sure that buttons or other working parts of the drill bit or bit at the outer end of the tool always hit a new spot in the rock. The rock drill machine generally comprises a hydraulically operated percussion device, whose percussion piston provides the tool with the necessary compression stress waves and a rotating motor that is separate from the percussion device. In the percussion method efficient breaking of rock requires that the bit be against the rock surface at the moment of impact. The impact energy of the percussion device strike produces in the tool a compression stress wave, which is transmitted from the tool to the bit arranged in the tool end and therefrom further to the rock. Generally, in all drilling conditions part of the compression stress wave reflects back to the tool as tensile stress. If the rock is soft and the rock/bit contact is poor the level of tensile stress is high in the wave reflecting from the rock. If drilling is continued into soft rock with excessive impact energy it generally results in worn threaded joints between the drill rods and/or premature fatigue failures of the drilling tool.
In general, the method that is currently used for drilling control, a so-called feed-impact-followup-control method, is not able to prevent drilling into soft rock with excessive impact energy. In the feed-impact-followup-control method the impact pressure is controlled on the basis of the feed of the drilling machine. The interdependence of the impact pressure and the feed pressure in rock drilling is presented in U.S. Pat. No. 5,778,990, for instance. When soft rock is drilled, the feed pressure remains in the set value. Only, if the velocity limit set for the feed of the drilling machine is exceeded, the feed pressure drops and the pressure of the impact along with it. However, in a situation, for instance, where the feed-impact-followup-control method is used for drilling from hard to soft rock, the penetration rate of the drilling rises. In practice, it is impossible to set the velocity limit of the feed to be sufficiently accurate for penetration rate values of different rock types, in order for the velocity limit of the feed-impact-followup-control to restrict the feed pressure in a desired manner. Because the penetration rate of the drilling thus remains below the velocity control limit set for the feed, the feed pressure and consequently the impact pressure remain at the original level, which results in high tensile stress in the tool. Generally speaking, the velocity limit is constant and it is set so high that it will not detect change in rock type, but only drilling into a void.